This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ion symporters are integral membrane proteins that co-transport two different ion species across the membrane. They play important roles in human physiology: the Sodium/Iodide symporter, for example, is responsible for accumulation of iodide in the thyroid gland. However, an atomic level mechanistic understanding of how symporters work is lacking. The long term goal of this research is to obtain high resolution structures of an E. coli Sodium/Potassium symporter, TrkH, in complex with a regulatory intracellular protein, TrkA. We will address the questions of how the ion selectivity is achieved, and how the function of ion transport is regulated by intracellular ATP level.